Add test cases for association object type many to many relations

As requested for https://github.com/jfinkels/flask-restless/issues/166

* Add models defining m2m relation
* Test posting a new m2m relation
* Test patching an object without any existing m2m relations
* Test patching an object with an existing m2m relation
* Test patching an object to remove a m2m relation

An association object many to many relation allows us to
have a normal many to many relation that holds extra data in the
helper table.

Notes:

We have two failing tests, [test_patch_m2m][tpm2m] and [test_patch_remove_m2m][tprm2m].  Both due to the same issue.

If you want to create a separate branch for this issue, let me know and I can do the PR again.

[tpm2m]: https://github.com/chrismckinnel/flask-restless/pull/new/tests-for-m2m#L1R413
[tprm2m]: https://github.com/chrismckinnel/flask-restless/pull/new/tests-for-m2m#L1R462